1.Field of the Invention
This invention relates to a process for producing a toner for developing electrostatic images.
2.Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, etc. are known as electrophotography. In general, using a photosensitive member comprising a photoconductive material, copies or prints are obtained by forming an electrostatic image on the photosensitive member, subsequently developing the electrostatic image by the use of a toner to form a toner image, transferring the toner image to a transfer medium such as paper if necessary, and thereafter fixing the toner image onto the transfer medium by the action of heat, pressure or heat-and-pressure. As methods for developing the electrostatic image by the use of toners or methods for fixing the toner image, a variety of methods have been proposed.
Toners used for such purpose have been commonly produced by melt-kneading colorants comprising dyes and/or pigments, into thermoplastic resins to effect uniform dispersion, followed by pulverization using a finely grinding mill, and the pulverized product is classified using a classifier to produce toners having desired particle diameters.
Fairly good toners can be produced by such a production method, but there is a limit to the range in which toner materials are selected. For example, colorant-dispersed resin compositions must be brittle enough to be pulverizable with ease by means of a fine grinding mill. However, such resin compositions made brittle in order to meet these requirements tend to result in a broad particle size distribution of the particles formed when actually pulverized at a high speed, especially causing a problem that fine particles tend to be produced in a relatively large proportion. Moreover, toners obtained from such highly brittle materials tend to be further pulverized in developing assemblies of copying machines or printers. Also, in the melt-kneading method, it is difficult to uniformly disperse solid fine particles of colorants or the like in the resin, and some toners may cause an increase in fog, a decrease in image density and a lowering of color mixing properties or transparency of toners at the time of image formation, depending on the degree of dispersion. Accordingly, care must be well taken when colorants are dispersed. Also, colorants may come bare to rupture sections of pulverized particles, and may cause fluctuations in developing performance of toners.
In order to overcome the problems of the toners produced by such melt-kneading and pulverization, toners produced by suspension polymerization are proposed as disclosed in Japanese Patent Publications No. 36-10231, No. 43-10799 and No. 51-14895, and also various polymerization methods and production processes for such toners are proposed. For example, in the process for producing toners by suspension polymerization, a polymerizable monomer, a colorant and a polymerization initiator, and also optionally a cross-linking agent, a charge control agent and other additives are uniformly dissolved or dispersed to form a polymerizable monomer composition. Thereafter, this polymerizable monomer composition is dispersed in an aqueous medium containing a dispersion stabilizer, by means of a suitable agitator, and is simultaneously subjected to polymerization reaction to obtain toner particles having the desired particle diameters.
Since this method has no step of pulverization, the toner particles are not required to be brittle, and hence soft materials can be used as the resin. Also, colorants may hardly come bare to the surfaces of toner particles, and hence the toner particles can have a uniform triboelectric charging performance. This method has such advantages. Also, since the toners obtained have a relatively sharp particle size distribution, the step of classification can be omitted, or, even when classified, toners can be obtained in a high yield. The method also has the advantage that, since a low-softening substance can be encapsulated in toner particles in a large quantity, the toners obtained have superior anti-offset properties.
When the polymerization toners are produced, it is essential to control temperatures of the reaction system in the mixing step of preparing a polymerizable monomer composition containing at least a polymerizable monomer and a colorant, in the granulation step of dispersing the polymerizable monomer composition in an aqueous medium by the action of a high shear force to bring it into particles and in the polymerization step of polymerizing the particles of the polymerizable monomer composition. Hitherto, as methods of heating the contents and keeping the temperature constant, it is common to use a method in which hot water is used as a heat source. Such hot-water heating, however, has problems such that it results in a poor productivity, requires a great cost to heat water, and makes it necessary to make great maintenance because of corrosion and accumulation of scale in a jacket. Also, in a method in which a jacket is filled with water vapor having a pressure of atmospheric pressure or higher, i.e., 100.degree. C. or higher saturated water vapor, the method can achieve a good temperature response, but the difference in temperature between the jacket and the materials to be treated is so great that the liquid temperature may vary in a great range to make it difficult to keep a constant temperature, tending to cause a lowering of product quality.
In addition, since deposits such as scale tend to be formed on the inner walls of the container used in the polymerization step, glass lining and fluorine resin are preferably used as inner wall materials for the container. Such materials have a poorer heat conduction than usual metallic materials, and hence a poor temperature response comes into question especially when the temperature control method employing hot water as a heat source is used.